Currently, there are unidirectional or bidirectional composite structural materials formed from SiC fibres and a glass mould and obtained by impregnating a fibrous preform with the aid of a bonding agent containing the elements required to obtain the desired vitreous composition followed by drying of the impregnated preform. Dried prepregs are then stacked and rendered integral with one another by means of moderate heating so as to obtain large composites with mono or bidirectional reinforcement, this preform being finally densified during a hot compression stage at a preferred temperature of between 1000.degree. and 1200.degree. C., as described in the document FR.A. 2 648 806. There are also vitroceramic compositions obtained via the sol-gel method and having a low coefficient of thermic dilation and able to withstand considerable and rapid temperature variations. The document FR-A 2 655 327 describes a vitroceramic composition whose principal compounds are SiO.sub.2 , Al.sub.2 O.sub.3 and LiO.sub.2 or MgO obtained via the sol-gel method from silicon and aluminium alcoholates and lithium or magnesium compounds, said composition mainly being in the form of a solid ceramic solutions whose low coefficient of dilation favors the formation of composite materials having good thermomechanical characteristics.
Owing to their inherent quality, the above-mentioned structural materials can advantageously be applied in the spatial and aeronautical fields. However, they are not high-performing as regards sound and thermic insulation. Thermic and sound insulating materials do nevertheless exist and these include glass foams with an aluminosilicate base and an expanded structure, as described in the document FR-A 2 578 828. This aluminosilicate, which is at least partially crystallized, results from oxidation of aluminum and silicon with (which of course contains nitrogen) resulting in obtaining an expanded alumino-silicate substance with closed pores containing nitrogen. This material is waterproof and has good resistance to chemical agents and constitutes a good thermic insulator. On the other hand, it only has mediocre structural qualities.
However, none of these products combines all the qualities required to be able to embody high-resistant and light bearing structures suitable for the production of panels, especially for aircraft and space vehicles which strictly need to have high-performance characteristics and at the same time possess sound characteristics relating to mechanical resistance, thermic insulation, sound insulation and with chemical neutrality, all these characteristics being also required for use at high temperatures, namely up to about 1000.degree. C.
Indeed, it would be possible to combine composite structural materials and thermic and sound insulating materials and thus obtain a sandwich structure having the sought-after characteristics. For example, by lining the walls with a mould of densified SiC/glass skins and introducing inside a mixture of glass powder and aluminium nitride as described in the document FR-A 2 578 828, an expanded glass foam linked to the walls would be obtained following an appropriate thermic treatment. The expansion of the foam exerts pressure on the walls of the mould which favors the linking with the SiC/glass skins. This apparently seductive solution does nevertheless have drawbacks causing the Applicant to seek another material and method. These drawbacks are mainly due to the difficulties encountered in manipulating the aluminium nitride and glass powder mixture to embody the moulds, even when this involves obtaining flat plates, and to have the glass foam expand so as to obtain thin and extremely slender pieces.